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Roar of triumph is a feature granted to member of the Barbarian class that choose the Thaneborn Triumph Feral Might option.

Roar of Triumph
Barbarian feature

Trigger: your attack reduces an enemy to 0 hit points

Target: each enemy in burst

Effect: "Each target takes a −2 penalty to all defenses until the end of your next turn." [PH2:50]

Your howl of victory shakes your enemies to the core, as they know your blood thirst is not yet quenched.